Optical fibers are generally cylindrical structures designed to transmit light along undulating trajectories; the light radiation propagates there by a series of total reflections or refractions which occur at the region separating the core of the fiber, which has a high reflective index, from its periphery which has a low refractive index. Rather than using a single fiber, it is often preferable to replace this by a bundle of fibers having the same overall diameter: the bundle is more flexible and less fragile than a single fiber and a break in one of the individual fibers does not bring about the total interruption of transmission. Bundles of this kind are generally constituted by several tens of strands each having a diameter of around 50 microns, assembled in mutual contact within a plastic sheath which lends the system mechanical protection. The bundle typically has a diameter of the order of half a millimeter in the bare state and of the order of two millimeters when sheathed.
The creation of detachable connections between bundles of fibers of this kind, nevertheless gives rise to problems of a tricky nature if the luminous flux losses are to be reduced to the minimum value. Direct contact between the entry and exit apertures of the fibers is out of the question because rapid deterioration in the surfaces occurs with successive dismantling and reassembly. On the other hand, the light beam coming from a narrow aperture thus exhibiting substantial divergency, the apertures must be arranged as close as possible to one another to avoid the interposition of a condenser lens between them. It is highly desirable, furthermore, in particular in the field of telecommunications, to utilise plug-in connectors of small size of a kind similar to those used in electrical connections.
To this end, it has been proposed that connection between two fiber bundles should be established by equipping the ends of these bundles with a ferrule of the same kind, comprising for example a female mechanical component; the connector proper is then constituted by an intermediate rigid component with two ferrules, (male elements in the chosen example, which penetrate into the aforesaid female ferrules) attached to the ends of a single fiber having the same diameter as that of the bundles which are to be connected. The single fiber mixes the luminous fluxes coming from the individual fibers in the emitter bundle and transmits a substantially uniform luminous flux to the apertures of the fiber constituting the receiver bundle.
This kind of connector has numerous advantages. It is much less bulky than devices which utilise convergent optical system. The efficiency of the connection does not depend upon the mutual angular positions of the two bundles as it occurs when the two bundles are arranged directly end to end (the most unfavourable position then being that in which the centers of the apertures of the emitter fibers coincide with the dead spaces between the apertures of the receiver fibers). Furthermore, in the case where the two bundles contain defective fibers, the undamaged fibers of the receiver bundle whose apertures are located opposite those of defective fibers in the emitter bundle are able to participate in the transmission function, which would not be possible with bundles directly arranged end to end.
However, if compared with the connections utilised in conventional electrical transmission arrangements, this solution has the drawback that an intermediate component which, because of its small dimensions, can be easily lost is required to connect the bundles.